app->mousepad: add cursor grab

IMPORTANT

• Depends on !411 (merged)

Adds relevant support to the kdeconnect-app.exe for cursor grabbing and remotely controlling cursor on a different device.

TEST PLAN

• Start kdeconnect-app and connect to a device supporting remote mouse input

• Click and hold left and right mouse buttons in the mousepad area of the remote input plugin on local device

• Notice the local cursor has been hidden away

• Notice remote cursor can now be controlled as if it is the local cursor.

• Currently supports:

  • leftclick
  • rightclick
  • middleclick
  • drag for selection
  • scroll

• Ideally, I think all of the clicks should be converted to explicit someMouseButtonDOWN and someMouseButtonUP calls. But it seems out of the scope of thisMR.