imcontextsimple: Ignore more modifiers in compose (gtk3)

( The gtk4-version of this patch is !3468 (merged) )

Previously it was impossible to compose characters on higher levels of some keyboard layouts as pressing the level selection key would just exit compose mode.

Examples for affected keyboard layouts include the Latvian apostrophe-variant "lv(apostrophe)" (latched third level), the extended German keyboard layout "de(e1)" (latched fifth level) as well as the multilingual Canadian keyboard layout "ca(multix)" and the German neo-layout "de(neo)" and its descendants (shifted fifth level).

To reproduce, set a compose key and select the Latvian apostrophe layout. Notice that you now can input [ by pressing first the ' and then the 8-key. Then pressing (compose)'8'8 should produce ⟦, but prior to this patch it did not.