Add items under "mesh" menu

[yungyuc]

For issue #324

Update the following items under "mesh" menu:

• Sample: 3D mesh of mixed elements
• Sample: NACA 4-digit

Add the following items under the menu. They are migrated from the module "modmesh.app.sample_mesh":

• Sample: mesh of a triangle (2D)
• Sample: mesh of a tetrahedron (3D)
• Sample: mesh of "solvcon" text in 2D

The new code is in the Python modules modmesh.gui.sample_mesh and modmesh.gui.naca.

A new C++ Qt class modmesh::RPythonAction is added for the menu items.

[yungyuc]

The coding style and encapsulation are decent,

but the example is not easy to read.

No. They are not easy to read. They are supposed to be viewed in the viewer.

[yungyuc]

I am going to merge. @j8xixo12 feel free to ask for modification. I will address in a future PR.