ShowImage: use BButton icon support.

* BIconButton is not needed anymore, now that BButton supports icons.
* Slight look changes. These buttons are a bit bigger, with extra
whitespace. I compensated this by reducing the insets and spacing, which
looks the same when no button is hovered, but a bit different when one
is, as the button frame is bigger. Maybe we need to tweak BButton to
have smaller margins like BIconButton did.
Revert attempt to fix stretch-to-window.

Turns out the option shouldn't do what I need. Let's add another option.
ShowImage: add feature to force original size.

* This is enabled by shift+click on the Original Size toolbar button.
* The button stays pressed as long as the feature is enabled. Clicking
it again disables it.
* When this is enabled, all images will be shown at original size,
instead of being scaled up or down to fit the window.
* This is not persistent, and only affects the current session/window.
Update some docs for the ARM port.

* Add info on Allwinner A10 SOCs, which may be a worthwile target.
* Add info about the Linaro QEmu for emulating the BeagleBoard.
* Add more TODOs for PM aftermath.