Browser request: Map Mode panel width

[adam maggs]

The panel width behaviour of the Mapping Browser interrupts the process of making assignments to tracks:
- all the browser windows open to display a uniform and equal panel width, set the last time any of the browser windows were resized
- the Mapping Browser is different, as it has it's own panel width, set the last time IT was resized

This means that activating MIDI Map Mode while the Browser is open results in a widening or narrowing of the Browser panel
- this also means that the tracks in Session will move to the left or the right
- and finally, the consequence is that whatever track parameter you are about to assign to moves from you at the very moment you want to grab it to make the assignment

This is a minor interruption in the scheme of things, but it's a constant one and it seems unnecessary.
Making the width of the Mappings Browser follow the same resizing as all the other browser tabs would smooth the workflow when making keystroke and midi assignments