Zooming in montage mode with default keybindings

[chrysn]

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In montage view, it's not possible to zoom around with default key bindings.

A key binding I'm currently using is

[keybindings]
@MONTAGE {
    + { set_thumbnail_size(256,256) }
    - { set_thumbnail_size(128,128) }
}

which I'd suggest to add into the default configuration.

This works for the two sizes that were originally (before #199) the only standardized ones, and is probably good enough. An alternative would be to add a set_thumbnail_size_relative command with default bindings of 2.0 and 0.5, which would stay on the standardized values for the reasonable sizes.

[phillipberndt]

The alternative was the first thing that came to my mind as well, but I agree the proposed version is probably good enough for now.