Add "pan with mouse" feature

[wawuwo]

Hello!

If merged this PR would add the ability to scroll schematic by moving mouse with middle button pressed down:
panning.webm

It also introduces a change that I'm not quite sure about: I had to delete setHidden(true) and setHidden(false) before and after resizeContents in renderModel method because the Q3ScrollView contents widget didn't receive MouseMove events after hiding and then unhiding the widget. I haven't observed any differences after removing these lines though. May be they don't make any sense now (they were inherited from legacy implementation) and it's OK to remove them.

[zergud]

thank you for contribution!

[ra3xdh]

@wawuwo , Thanks for the contribution! It would be good to place a "Pan" button on the toolbar near to "zoom" buttons. This will make the new feature visible for users.

[zergud]

@ra3xdh pan by pressed down middle button does not require button in toolbar
it is "well known" behavior for EDA end CAD users

[wawuwo]

@ra3xdh

It would be good to place a "Pan" button on the toolbar near to "zoom" buttons

Yeah, I see what you mean, but unfortunately it cannot be done just by adding a button :(

If I have correctly understood you, the "Pan" button you mean assumes that by clicking on it the user switches to a "panning mode", which has its own mouse event handlers; and when the user is done panning the previous "mode" has to be restored. I believe I know how to implement this in general, but I'm afraid diving into details may take some time.

If this feature is really needed I think I will be able to work on it, but as @zergud wrote, "press mouse wheel to pan" is a common approach. Hence maybe there is no need for separate panning mode/button and users will discover the feature intuitively by themselves

[ra3xdh]

but unfortunately it cannot be done just by adding a button :(

If it requires a sufficient effort to implement, the task may be omitted. The current implementation will be sufficient.