Revert "Remove KisTool::notifyMdified"

This reverts commit 8ef85fdd39aef257ae03a4fe07e9d8c92003e177.

This commit fixes issue with autosave only saving once per manual save.
Possibly it also fixes the issue with editing time being way lower
than expected.

Mentioned commit removed sending signal KisImage->notifyModified()
by KisFreehandTool. Turned out that signal was important for autosave
to work correctly and _possibly_ for counting time spent on the painting.

Theoretically this signal should be also sent by strokes, rendering
sending signal here redundant (hence the previous commit with
"It is not used anymore" message). This requires further investigation
and then recommitting these changes after making sure all logic is
performed as before.

Looks like the course of action with the signal from the Freehand tool was
as follows:

Freehand Tool sends ModifiedSignal

-> Image Signal Router sends sigImageModified()
-> KisDocument catches it and calls setModified( undoStackNotEmpty )
-> KisDocument does a lot of logic and then sends modified()

Supposedly strokes call KisDocument::setModified(...), at least on master.
I can't confirm it because I didn't check the relevant code yet.
But if that's true, that should be enough - all relevant logic happens
inside this function. On the other hand, practice seems to prove
otherwise...

BUG:411631