Add filter buttons and filter textbox to all tabs

[Jonathan Thomas]

Each tab in OpenShot (Project Files, Transitions, and Effects) needs to have a way to filter and search for the required item. Here is what I propose:

Project Files:
Toggle buttons for quick filtering: Show All, Video, Audio, and Image
Textbox: As the user types, the list is searched for matching filename's and labels

Transitions:
Toggle buttons for quick filtering: 'Show all' and 'Common'
Textbox: As the user types, the list is searched for matching transition names

Effects:
The effect tab already has filter toggle buttons and a textbox.