Re-added GUI files

[davidpagnon]

Hi @AYLARDJ,

I messed up with your pull request #180, so I have to reopen one. Sorry about this! And even more so, I realize how long this pull request has been on... First of all, this is a very great job, thank you so much! This is quite a big thing to integrate, and your interface actually gave me tons of other ideas, that (guess what?) I haven't found time to implement.

Here is what I currently did:

• Added the logo to the Intro and Welcome screens
• Created dark/light mode switch button, added it to all tabs
• Configured language switch button, added it to all tabs
• Tabs are loaded when clicking on them rather than all from the start, which was slow
• Fixed the checks of Anaconda path and OpenSim install, made it 3 rows of 2 columns, placed OpenSim before Pose2Sim (my lead to bugs in the other order)
• Uniformized layout (in particular, fonts)

Here is what I want to do next:

• Load configuration file instead of hard-coding it. I'm afraid that I'll forget to reflect Config changes in the GUI. I also display the comments when hovering over each parameter. This makes it slow, though, so I'm thinking of loading the configuration on each tab instead of having a big tab with all the advanced configuration
• Complete translation
• Presented the GUI in the Readme.md file
• Merge language, analysis, welcome, and tutorial windows (Intro, and then straight to 2D/3D, single/batch, project name/load config)

Here are some things that are not very important, and that I did not manage to do fast, so that I left as undone:

• Grab border between marker / plot areas to resize. Add zoom / pan controls on plots
• Between intro.run() and root = ctk.CTk(), it seems like the Ctk window is not trully destroyed, which leads to the following type of errors:invalid command name "3025061505728update" while executing "3025061505728update" ("after" script). It does not have any impact on running the GUI other than the error message.
• Tried to set a favicon (for the taskbar and for the title bar), but it did not seem to work.
• Tried and failed to embed the tutorial videos instead of using a link.
• Push buttons to the bottom of the frame and place them in the same location from tab to tab. This is surprisingly difficult