Resize app window to match aspect ratio with middle click

[Rafostar]

Add an action to resize window into a nearest best fit for currently displayed video in order to remove black bars.

This approach has multiple benefits:

• Easy and fast to perform
• Does not break tiling compositors
• Does not require additional options in preferences to enable/disable constraint
• Avoids situations where multiple videos are played in order with different aspect ratios and window keeps shrinking/growing on this own with every clip (and does not go back to earlier size)
• Optional and thus still allows to resize window into a size where OSD does not cover video area

Related to #30