Allow hiding left pane

[GoogleCodeExporter]

When picking a palette from screen or image, the more space for the image, the 
better. Hiding the left-pane(which is irrelevant to this task), allows the 
GPick window to be much smaller, and also allows you to move the right edge of 
the window offscreen while keeping the palette colors visible (effectively 
reducing the space occupied by GPick to little more than the width of a palette 
color).

A very simple demo of what this could be like can be achieved by simply 
inserting a line `gtk_widget_hide(notebook);` after the 
`gtk_widget_show(statusbar);` in uiApp.cpp.

This simple test shows a few things:
* the palette widget needs to be reparented, much as the notebook currently is 
when the palette view gets toggled.
* Picking with Space no longer works (instead, it 'selects the current palette 
item', an operation which has no useful effect when the main pane is hidden). 
This is important. 
* The 'Secondary view' submenu needs to be disabled when the main pane is 
hidden.
* The 'Palette' toggle also needs to be disabled (it's pretty silly to have a 
window that is just empty.)

Original issue reported on code.google.com by fintic...@gmail.com on 13 Jan 2014 at 8:38

[GoogleCodeExporter]

^ actually, I take that back about the palette visibility. It is actually, 
surprisingly, still quite useful as long as you can pick colors with Space 
and/or Ctrl+P, and see the color count.

Original comment by fintic...@gmail.com on 13 Jan 2014 at 8:41