Improve the mechanism of installing/uninstalling plugins of QtCreator.

Personally, I usually write C/C++ codes with the following 3 IDEs:

• VSCode
• CLion
• QtCreator

And all of them have their own plugin ecosystem:

• VSCode: Visual Studio Marketplace
• CLion: JetBrains Marketplace
• QtCreator: Qt Marketplace

If I want to install their plugins, then I can go to the above marketplaces to download and install them manually.

Both VSCode and CLion provide a GUI dialog for plugins so that I can search and install plugins inside the IDE, which is really convenient. 

It seems that QtCreator provides the "Search" feature for Qt Marketplace. However, we still need to download the binaries of plugins and "install them to specified path manually" ourselves. I think this is one of the reason why QtCreator's Plugin Ecosystem cannot flourish as well as VSCode and CLion did.

Therefore, I hope QtCreator can improve its mechanism of "Installing/Uninstalling" its plugins.

Concrete Suggestions

1. Put Installed Plugins and SearchBar together

As we can see:

• The "installed plugins" are displayed in the dialog opend by clicking "Help -> About Plugins...".
• The "searchbar" of marketplace is displayed in the page of "Welcome -> Marketplace".

Take VSCode for reference. Maybe they can be put together into an individual page of the sidebar.