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. They are not part of plugintemplate

References:

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

[AntonDevil]

This is an alternative to #222