Fix/fractional scaling and tiling bugs

[ScherbakovAl]

Problem

Since GNOME 49, fractional scaling (logical monitor layout mode) is enabled by
default on Wayland and is no longer listed in mutter's experimental-features.
The extension therefore assumed it was disabled and applied the monitor scale
factor on top of the compositor's own scaling. At 150% scale this made the
indicator menu, snap assistant, layout editor and tile previews 1.5x too big
(the menu did not even fit the screen) and the window border too thick and
misplaced.

While debugging this, I also found a few unrelated tile geometry bugs.

Fractional scaling fixes

• Add isFractionalScalingEnabled() helper that treats fractional scaling as
  enabled on GNOME 49+ Wayland sessions (with a fallback to
  XDG_SESSION_TYPE, since Meta.is_wayland_compositor() was removed in
  newer GNOME versions).
• Fix inverted enableScaling flag when recreating WindowBorderManager on
  experimental-features changes.
• Gate the snap assistant threshold and the overridden window menu scaling on
  actual scaling support instead of applying the monitor scale factor
  unconditionally.

Tile geometry fixes

• findNearestTile used source.x for the y coordinate of the window
  center, selecting a wrong tile when moving across monitors.
• Missing parentheses in the "best tile" distance computation in auto-tiling
  and in the overridden window menu.
• Use innerGaps.top (instead of .right twice) when enlarging the search
  area for keyboard-driven tiling.
• Use ?? instead of || for the monitor index fallback, so monitor 0 is not
  mistaken for the primary monitor.

UI fixes

• Wrap the editor dialog's layouts row in a horizontal St.ScrollView so the
  dialog fits the screen with many layouts or low resolutions (mouse wheel
  scrolls horizontally).
• Give the preferences window a proper default size (720x940, clamped to 90%
  of the monitor size) instead of opening smaller than its content.

Tested only on my own setup: GNOME Shell 50.3, Fedora, Wayland, with
fractional scaling. Other configurations (X11, older GNOME versions) were not
tested — review of the version-gated paths is appreciated.

** Generated by Fable 5. I hope the corrections are correct.