Migrate from copies from external plugintemplate to git submodules

[AntonDevil]

Currently, JSONViewer includes files from plugintemplate directly copied into external/npp
This PR removes these copies and migrates these dependencies to git Submodules like all other existing dependencies

Updates cause by move to submodules:

1. Added plugintemplate into .gitmodules
2. Added proxy files (forwarding headers/sources) to \src\NppJsonViewer\plugintemplate folder for ease of use
   as done in Docking_Dialogue_Interface alternative template mentioned in official How to develop a plugin
3. Updated ExternalIncludePath in NppJsonViewerCommon.props to include new NppJsonViewer\plugintemplate folder
   $(SolutionDir)..\external\npp; -> $(SolutionDir)NppJsonViewer\plugintemplate;
4. Fixed includes to follow new folder structure because some files are now in DockingFeature subfolder
   Updated in: AboutDlg.h, JsonViewDlg.h, SettingsDlg.h
5. Sorted files in NPPJSONViewer.vcxproj and NPPJSONViewer.vcxproj.filters in lexicographical order (ignoring case)
   Moved plugintemplate\ files to the bottom since they are not a direct part of main project

Updates cause by plugintemplate update:

1. Replaced DEPRECATED NPPM_ADDTOOLBARICON with NPPM_ADDTOOLBARICON_FORDARKMODE in NppJsonPlugin.cpp reusing same m_hMenuIcon.hToolbarIcon for toolbarIconsWithDarkMode for dark mode
2. Replaced DERPECATED NPPM_GETENABLETHEMETEXTUREFUNC with direct call to EnableThemeDialogTexture in SettingsDlg.cpp
3. Added uxtheme.lib as AdditionalDependencies to NPPJSONViewer.vcxproj to link newly added EnableThemeDialogTexture
4. Removed! unused URLCtrl.h and URLCtrl.cpp

References:

• JSON-Viewer
• Submodules
• How to develop a plugin
• plugintemplate
• Docking_Dialogue_Interface
• notepad-plus-plus

[AntonDevil]

This is an alternative to #222