Improved label painting in BandDock

[ypnos]

Right now, in the "Label" mode the user has a one-pixel cursor to change labelings. The left click action is add to label, right click is remove from label. However we want the right click to be usable for the new context menu from #14.

This enhancements consists of several parts.

1. Enhance the ModeWidget to contain more controls when in Label mode. This is similar to the ProgressBar enhancement of the control widget in FalseColorDock. See falsecolor_sel.ui and how it is used for inspiration.
2. In the new part of the widget, include the following items:
   • Several buttons depicting different cursor sizes to choose from
   • A rubber button which makes the cursor remove labeling (set label to zero) for all pixels of same label, shortcut 'x' for this button
3. Implement different cursor sizes for drawing in the bandview. Ideally, the cursor would consist of a mask like this:

cursor 1:

1

cursor 2:

0110
1111
0110

Cursor 3:

0011100
0111110
1111111
1111111
1111111
0111110
0011100

When drawing labels, three things happen:

• the label is altered
• the bandview is updated to reflect that
• a list of changed pixels is amended After a while, the list of changes is submitted so that the viewports get incremental updates to their binning. This needs to be taken into account when making the change.

[ypnos]

Merged pull request #18