Paging desktop offset to centre of screen

[felixfung]

This is continuation of #157. @dreamcat4 please test

the paging layout is rectangular such that screenwidth == ceil(sqrt(screencount)) and the screenheight == ceil(screencount / screenwidth)

[dreamcat4]

• i am going to merge this, then test this pr today. since it is easy one to test. is not much burden here.

• but i am not going to be testing you other future prs, until after a future time that i have then worked on and fixed for the multi monitor support (the priority task)

• however (for the next times) i can still do some lighter code review, before merging

  • but only when those code is actually touching or in a part of skippy that
  • i am familiar enough to understand
  • so you might instead be smart enough to go ahead by yourself to merge for any other pr, for that is not going to benefit here from peer code review. or that is out of my scope (for example other future paging feature fix / improvements). other user bugfix, and so on

i hope this clarifies here. because it's adhoc / not so formal working arrangements. to keep both on same page

[dreamcat4]

(code was rebased)

[dreamcat4]

testing result:

Previous behaviour (before merge):

• the group of (5) selectable desktop previews appeared from the top of the screen (of the mainwin, full skippy window visible area)

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The new observed behaviour:

• the group of (5) desktop previews are now:
  • center aligned (vertically) to appear in the middle of the active monitor.
  • No other changes detected, in the general observed behaviour

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So I don't see any issue or unindended consequences here with this. All seems fine.