gtr-tab.c: Save window's layout when close

Merged Amanda Shafack requested to merge lkmandy/gtranslator:save_layout into master

When the main window is closed, the layout of the vertical and horizontal panes are set to their default values, even if their positions had been changed by a user.

Use Gsettings to save and restore the layout of the panes.

Fixes #95 (closed)

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