It opens the files selector if "filename" property has not been set, else it executes the operator directly. Brecht, you might want to check, currently only sequencer add operators use it.
* Added back Effects menu back, replaced SEQUENCER_OT_add_color_strip with SEQUENCER_OT_add_effect_strip.
* Made sequencer header use operator UI functions.
* gcc complains when char's are used as array indicies when they are not explicitly signed/unsigned, corrected previous change for vpaint to silence this error.
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