Allow users to use their own Discord application (fix #66)

[echebbi]

Allows users to change the icon and the name of the game shown in Discord (fix #66).

Both IDE and project preferences pages have been enhanced with a new "Icons" subsection to allow the use a custom Discord application ID:

Moreover, a part of the core have been refactored

• the code responsible of updating Discord upon preferences change has been moved from FileChangeListener to the new UpdateDiscordOnSettingChange class
• FileChangeListener has been renamed as UpdateDiscordOnActiveEditorChange
• some packages have been renamed and classes moved

[codecov[bot]]

Codecov Report

Merging #69 into master will decrease coverage by 2.24%. The diff coverage is 70.52%.

@@             Coverage Diff              @@
##             master      #69      +/-   ##
============================================
- Coverage     74.92%   72.67%   -2.25%     
- Complexity      200      207       +7     
============================================
  Files            23       25       +2     
  Lines           630      732     +102     
  Branches         62       78      +16     
============================================
+ Hits            472      532      +60     
- Misses          145      179      +34     
- Partials         13       21       +8

Impacted Files	Coverage Δ	Complexity Δ
...u/discord/rpc/integration/core/SelectionTimes.java	100% <ø> (ø)	8 <0> (ø)	:arrow_down:
...ntegration/extensions/EditorInputRichPresence.java	100% <ø> (ø)	1 <0> (ø)	:arrow_down:
.../discord/rpc/integration/files/OnPostShutdown.java	0% <ø> (ø)	0 <0> (?)
...rd/rpc/integration/core/PreferredRichPresence.java	100% <ø> (ø)	11 <0> (ø)	:arrow_down:
...c/integration/files/AddListenerOnWindowOpened.java	100% <ø> (ø)	11 <0> (?)
...cord/rpc/integration/core/DiscordRpcLifecycle.java	0% <0%> (ø)	0 <0> (ø)	:arrow_down:
.../discord/rpc/integration/core/DiscordRpcProxy.java	0% <0%> (ø)	0 <0> (ø)	:arrow_down:
...tegration/settings/ProjectPreferencesListener.java	0% <0%> (ø)	0 <0> (ø)	:arrow_down:
...d/rpc/integration/settings/ProjectPreferences.java	0% <0%> (-7.15%)	0 <0> (-1)
...iyu/discord/rpc/integration/settings/Settings.java	97.22% <100%> (+1.98%)	5 <2> (ø)	:arrow_down:
... and 13 more

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[echebbi]

I sadly don't have enough free time to write as much tests as I'd like and this feature is waiting for a long time, so let's just merge now.