Add Screenshot button to Activity

[birbilis]

Some of the controls have a Screenshot button on their titlebar, however the Activity control doesn't have such (where it would be mostly useful)

[birbilis]

btw, at Window options, grouped under "Toolbar Visible" for all controls there should be a "Screenshot Visible" toggle (and other toggles for any buttons that can be shown/hidden).
Default value for "Screenshot visible" may differ per control class, for Activity one it should probably be true by default