fix: preview panel + main window fixes and optimizations

[CyanVoxel]

Summary

This PR provides a few fixes and optimizations regarding refreshing the preview panel and resetting the state of the main window UI as a whole when closing or switching libraries:

• The media file preview for the preview panel now only updates when necessary
  • Before this would always update when information about the entry was modified, including when tags were added or even simply edited. Now operations such as adding tags no longer cause image thumbnails to re-render or for videos to restart.
• The preview panel is now cleared when closing or switching libraries
• The search bar is now cleared when closing or switching libraries
• The scrollbar position is now reset when closing or switching libraries
• The filter state, notably the page number, is now reset when switching libraries