Main Menu: UI Improvements

[suhrmann]

Short and concise description about my pull request:

• Make dialog "Choose a login method" closable.
• Set "25555" as default value for port input
• Load the server list async and add (simple) loading animation.
• Reword "login" with "connect"

TODOs:

• - I confirm this PR is at its final form and will not receive more pushes to it unless modifications are required.
• - I confirm this PR has been previously tested by me and has no apparent issues.
• - I confirm this PR is complying with this project's Contribution Guidelines.
• - I confirm this PR is complying with this project's Syntax Ruleset.

Longer / More informative description about what my pull request does:

• Make dialog "Choose a login method" closable:
  Implement closable RT_Dialog_YesNo via X button by adding argument closableX that adds the X button to the top right that adds closes the RT_Dialog_YesNo with Window.doCloseX.

• Set "25555" as default value for port input in ConnectionManager:
  Implement default values for RT_Dialog_Inputs by adding argument defaultValues that, if set, initializes Results with default values.

• Load the server list async and add (simple) loading animation:
  Added the method GetServersAsync() that sets IsLoading, while loading. When server list is fetched, IsLoading = false and the server list is displayed.
  🚧 I also added a close X button while fetching servers, that does currently nothing. I fear problems closing the dialog without properly stopping the server search in the background.

• Reword "login" with connect:
  Changing only strings. Though, when I read "Login" I would expect username/password, so I think "Connect" fits better.

[Heibert]

I think there is an excellent improvement, please check it!