editor: add edit bookmarks action and dialog

Right now bookmark management in Lokalize is limited to toggle/previous/next actions. That works for quick marking, but it becomes hard to review or clean multiple bookmarks at once, especially in larger files. There was no dedicated UI to inspect all current bookmarks and edit them in one place.

This MR adds a proper Edit Bookmarks flow in the Editor tab. KStandardActions::EditBookmarks is now wired into the existing Bookmarks section and implemented as a dialog that lists all currently bookmarked entries with a short source-text preview. From there, users can uncheck individual items or use Clear All and apply changes with OK. Double-click navigates to the selected entry, Enter navigates the current list item.

Files touched:

• src/editortab.cpp: hook up KStandardActions::EditBookmarks in setupActions(), add editBookmarks() dialog and applyBookmarkEdits() helper
• src/editortab.h: forward declare QListWidget, declare editBookmarks() slot and applyBookmarkEdits() private helper