Add Native File Chooser Dialog integration for Nemo and XDG Desktop Portals

[Twilight0]

Overview & Rationale

Currently, Cinnamon environments running sandboxed Flatpaks or host applications rely on basic or external file chooser portals. These defaults lack visual coherence with the rest of the Cinnamon desktop and Nemo.

This Pull Request implements a custom, fully-featured native file chooser dialog for Nemo, exported over D-Bus via the org.Nemo.FileChooser service. When paired with the accompanying changes in xdg-desktop-portal-xapp, this allows all application file-picking requests (from GitKraken, Web Browsers, Flatpaks, etc.) to use a premium, native-feeling Nemo file selection interface.

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Features Implemented

1. Unified Multi-Pane Layout

• Places Sidebar: Integrates the standard Places sidebar (GtkPlacesSidebar), ensuring user bookmarks, network mounts, removable media, and recent paths are instantly accessible.
• Main Browser Stack: A dynamic layout supporting three view modes:
  • Grid View (Icons): High-fidelity layout with custom large icons.
  • List View (Columns): Standard layout showing file details.
  • Compact View: Horizontal icon flow for rapid list traversal.
• Dynamic Preview Pane: A sidebar on the right displaying real-time previews for selected files, including:
  • Image/video metadata (dimensions, size).
  • High-quality thumbnail previews retrieved asynchronously via the Freedesktop.org thumbnail cache specifications.

2. Directory Selection (SELECT_FOLDER / GTK_FILE_CHOOSER_ACTION_SELECT_FOLDER)

• Targeted Folder Listings: When operating in directory-selection mode, files are automatically hidden from the main browser list so users see and navigate only folders.
• Action Button Label: The main accept action dynamically switches its label to _Select rather than _Open.
• Hierarchical Fallback Selection: If a user is inside their target directory (e.g. /home/user/Projects/nemo) and clicks Select without highlighting a specific sub-folder, the dialog correctly falls back and returns the URI of the current folder.

3. Navigation & Context Enhancements

• Up Button: Placed on the header bar to navigate to the parent folder. Its sensitivity is dynamically calculated (disabled at the system root /).
• Interactive Location Bar: Switches between a clickable breadcrumb path and a text entry box via the edit button or Ctrl+L shortcut. Supports Escape to return to breadcrumbs.
• Create Folder Modal: A dedicated button on the header and options in the context menu to create folders instantly.
• Header Sorting (List View): Columns (Name, Size, Type, Date Modified) support clicking to sort. Folders are always grouped before files. Sorting is paused during list loads to avoid UI freezes.
• Column Customization: Right-clicking the List View header exposes a context menu to toggle the visibility of the Size, Type, and Date Modified columns.
• Context Menus: Right-clicking the file browser area provides options to toggle hidden files, customize sorting options, refresh the directory, copy locations, and create folders.
• Keyboard Shortcuts: Fully maps Ctrl+H to toggle hidden files and Ctrl+L to edit the location path text.

4. D-Bus Daemon Integration

• Updated the org.Nemo.FileChooser D-Bus XML specification to support folder selection:
  • OpenFile method signature updated to (sasbbs) to accept the directory boolean.
• Fixed the dialog response matching in nemo-file-chooser-dbus.c to accept both GTK_RESPONSE_ACCEPT (-3) and GTK_RESPONSE_OK (-5), resolving empty return values when using the Glade-defined ok_button.

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Testing Verification

1. D-Bus Interface: Invoked using custom Python test scripts calling the desktop portal.
2. Directory Picking: Verified directory selection returns the correct path structure to the calling process.
3. Application Test: Verified GitKraken (host Electron app) successfully uses this dialog to open local repositories via the portal.