fix(theme): changing theme breaks screens #531

[ryjen]

MainActivity

• upgraded the viewpager to google's viewpager2

MainMediaFragment;

• moved broadcast listener from activity
• moved delete options menu item from activity

ProjectAdapter:

• changed to viewpager2 adapter base class
• moved some item logic from activity

[uniqx]

I looked through the diff and the code changes look fine. I especially appreciate getting rid of mCurrentFragment 👍
I also did a quick build and clicked through the use-case a little, works great.