WIP Menus: Layout in menu-bar tweaks to lay better when mixed widgets (#126)

WIP Menus: Layout in menu-bar more standard, allows mixing different widgets in menubar. (#126) ...

Still some minor spacing bugs.

Selectable: text baseline alignment for line that aren't of text height.

WIP Menus: Sub-menu overlapping amount not hardcoded (#126)

WIP Menus: Fixed style.WindowPadding == 0 leading to zero-sized initial clipping rectangle leading to self-collapsing childs (#126)

Combo: empty label doesn't add ItemInnerSpacing alignment, matching other widgets

WIP Menus: Sub-menus appears over the parent-menu scrollbar if there is one (unsure about that) (#126)

WIP Menus: Fixed estimation of scrollbar width (broke in 292f08b) (#126)

WIP Menus: Fixed repositioning of menus when there's no room in the 4 quadrants., we still keep them within the visible display area (#126) ...

vs popup which prefer to stay away from the mouse cursor

WIP Menus: Clicking the label of an already open sub-menu doesn't close it unless from a menu-bar (match Windows behavior) (#126) ...

Argh,

WIP Menus: Fixed closing popup on menu item activation when a child menu is open from the popup (#126)

WIP Menus: Fixed manually closing submenu affecting position of the following window (#126)

WIP Menus: Fixed some alignment issues with non standard styles. Tweaked menu-bar color (#126)

Merge remote-tracking branch 'origin' into 2015-05-menus ...

Conflicts:
	imgui.cpp

Added Dummy() helper

WIP Menus: Added Enabled/Disabled option for MenuItem() (#126)

WIP Menus: Added BeginMainMenuBar()/EndMainMenuBar() helpers. Added to examples.

WIP Menus: don't clamp windows within display when a position has been explicitly set by user. Menu item spacing uses ItemSpacing. (#126)

WIP Menus: menus now affected by WindowMinSize (#126)

Merge remote-tracking branch 'origin' into 2015-05-menus ...

Conflicts:
	imgui.cpp

Removed unnecessary PushTextureId/PushClipRect/PopClipRect/PopTextureId on multiple Begin/End pairs

ImDrawList: merge draw command better, cases of multiple Begin/End gets merged properly

Fixed appending multiple times to an existing child via multiple calls to same BeginChild/EndChild

Removed a few empty lines

Updated FAQ for usage of ## to display empty label

InputText: Fixed cursor generating a zero-width wireframe rectangle turning into a division by zero.

WIP Menus: fixed recycling menu level during the same frame (#126) ...

We could also enforce "clearing" the window and recycle immediate which
sort of work, but it would be a less tested code path.

WIP Menus: tidying up, fixed hovering in parent popup menu item from child popup (individual popups acts as hovering barrier) (#126)

WIP Menus: click again to toggle + allow hovering in same menuset even outside of a menubar (#126)

WIP Menus: menu opens on press (not release), uses popup style hovering, allow switch between menus at parent site (#126)