Add alt to default colour channel keybinds

[Stoppedpuma]

PRing into dynamicimage instead of pushing since this wasn't discussed with anyone. Part of solving the confusion at #561.

Changes the default keybind for colour channels to require alt to reduce the chances of accidentally changing channels, but still being able to change channels with one hand.

[woelper]

I am not sure to what this would map on OSX since there is no alt, maybe it is mapped to option? I am concerned that this makes it hard for some folks to access the shortcut.

[Stoppedpuma]

I would assume option would be alt as that's what I've always found the equivalent to be? We could always do shift or just not change anything at all, I went with alt originally since people are less likely to set an equivalent keybinding to those keys combined with alt.

Upon further research it seems some mac keyboards actually label the option keys with alt, it seems to be more common in US layouts.

One more thing which is probably worth considering is if this would be handled correctly with reimplementing keybinds the way you wanted to (notes of discussion can be found at #408). This will likely have to happen at some point as inputs are still being dropped when multiple at a time are used, as well as stuck in some scenarios, relevant issues at #374 and #105. This bug is still very annoying on my end at least so maybe something to shoot for in 0.9.3 / 0.9.4?