Dedicated cursor for edit mode

[theGreatWhiteShark]

We need a better way and consistent way to distinguish (mouse) cursor-based drawing and selecting.

Within the Song Editor we can choose between two different modes: one in which click-dragging draws grid cells and another in which click-drag selects a range of grid cells. Right now, only the state of the corresponding button indicates the current mode. At least I do frequently end up doing something wrong because I assumed the other mode to be active.

In the Note Properties Ruler we have two modes as well: when starting a click-dragging at a note property, the mouse cursor draws new values for all notes it touches. When starting it at column without a note, click-dragging results in a lasso selecting a range of properties. That's a little bit annoying. At least during click-drag - but ideally when hovering too - we should use a dedicated cursor to indicate whether the action results in drawing or selection.

This task requires some intuition for graphics and design and is ideal for people who want to contribute to the project without having to introduce themselves into C++ and Qt.

The public domain (!) cursor should be 32x32 pixel (as expected by Qt). All you need to do is to post the image and a member of the Hydrogen dev team will take care of building an artifact you can use for testing.

An example of would could possibly be done are the selection cursor in Gimp when using key modifiers. But we are open for all sorts of ideas.