Extend input method check for gcin

[changhc]

Following #470 and #530, this PR extends the input method check to cover any input method that is not ibus.

In my case, I'm using gcin, an input method for Traditional Chinese. This is not covered in the current check, so xournalpp crashes every time I try to add some text to my pdf documents. My change will make the check more comprehensive than checking only against xim.

Edit: checking against xim and gcin only now.

Test output

$ echo $GTK_IM_MODULE
gcin
$ ./src/xournalpp
** Message: 18:15:22.754: TEXTDOMAINDIR = (null), PACKAGE_LOCALE_DIR = /usr/local/share/locale, chosen directory = /usr/local/share/locale            
                                                                                                                                                      
** (xournalpp:18484): WARNING **: 18:15:22.754: Unsupported input method: gcin, changed to: ibus                                                      
ALSA lib pcm_dmix.c:1089:(snd_pcm_dmix_open) unable to open slave                                                                                     
ALSA lib pcm.c:2642:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.rear                                                                                
ALSA lib pcm.c:2642:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.center_lfe                                                                          
ALSA lib pcm.c:2642:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.side                                                                                
ALSA lib pcm_route.c:869:(find_matching_chmap) Found no matching channel map                                                                          
ALSA lib pcm_oss.c:377:(_snd_pcm_oss_open) Unknown field port                                                                                         
ALSA lib pcm_oss.c:377:(_snd_pcm_oss_open) Unknown field port                                                                                         
ALSA lib pcm_usb_stream.c:486:(_snd_pcm_usb_stream_open) Invalid type for card                                                                        
ALSA lib pcm_usb_stream.c:486:(_snd_pcm_usb_stream_open) Invalid type for card                                                                        
ALSA lib pcm_dmix.c:1089:(snd_pcm_dmix_open) unable to open slave                                                                                     
** Message: 18:15:22.788: Plugin "MigrateFontSizes" UI initialized 

[tattsan]

This PR seems to exclude all input methods other than ibus. However I don't use ibus on Xournalpp, but uim. It is violent to "eliminate everything except ibus", isn't it?

[LittleHuba]

I don't really get the point of this PR as it moves from blocking one specific input method xim) to blocking all input methods (including xim).

So it does obviously not really improve the situation of the author, but worsens the situation of everybody else not using gcin?

Maybe instead help us to support gcin or block this one additional input method.

[changhc]

You're right. I didn't know other input methods work normally with xournalpp. I'll change this PR to block gcin only.

[personalizedrefrigerator]

I've applied clang-format!

This looks good to me! Merging in 24 hours unless there are objections.