Set focus better on Ctrl+F

[HebaruSan]

Problems

• Ctrl+F is supposed to focus the search box, but currently it doesn't
• Even if we fix the broken Ctrl+F handler, it will only work when the mod grid has focus, not mod info or any of the other parts of the UI

Cause

• Going back to a3d96f6, the focus has been set solely by setting ActiveControl, which doesn't always work (details unclear, but possibly related to the search control containing sub-controls?)
• Ctrl+F is currently handled by ManageMods.ProcessCmdKey, which is only called when that control has focus

Changes

• Now .Focus() is used to set the focus
• Now the handler for the Ctrl+F keystroke is moved from ManageMods back to Main so it can be global
  • Now ManageMods implements a new ISearchableControl interface to advertise its ability to respond to Ctrl+F keystroke, which Main then uses to find a control
• Courtesy of a VSCode performance-improvement suggestion, many usages of .Any() are replaced by checking .Count instead

After this, Ctrl+F sets the focus to the search box whenever it's visible, and if other controls (Edit Modpack, Unmanaged Files, etc.) add their own search tools in the future, they can implement ISearchableControl.