This commit rewrites selection handling at the TermControl layer.
Previously, we were keeping track of a number of redundant variables
that were easy to get out of sync.
The new selection model is as follows:
* A single left click will always begin a _pending_ selection operation
* A single left click will always clear a selection (#4477)
* A double left click will always begin a word selection
* A triple left click will always begin a line selection
* A selection will only truly start when the cursor moves a quarter of
the smallest dimension of a cell (usually its width) in any direction
_This eliminates the selection of a single cell on one click._
(#4282, #5082)
* We now keep track of whether the selection has been "copied", or
"updated" since it was last copied. If an endpoint moves, it is
updated. For copy-on-select, it is only copied if it's updated.
(#4740)
Because of this, we can stop tracking the position of the focus-raising
click, and whether it was part of click-drag operation. All clicks can
_become_ part of a click-drag operation if the user drags.
We can also eliminate the special handling of single cell selection at
the TerminalCore layer: since TermControl determines when to begin a
selection, TerminalCore no longer needs to know whether copy on select
is enabled _or_ whether the user has started and then backtracked over a
single cell. This is now implicit in TermControl.
Fixes#5082; Fixes#4477
499f24a29e