New snippets for Verilog and some reorganization 2

[mpkopec]

This is a partial revival of #1522.

Addressing some of the remarks from the previous PR.

• I have undone the reorganization of the existing snippets.
• No snippets have been moved to the UltiSnips directory.

A brief description of changes made this time:

• I have added some VISUAL markers, wherever the snippet operated on an begin-end like block, i.e. it is supposed to work with multiple lines of code.
• I have moved the comments above the snippets into their respective description fields; fixed a few typos.
• I have moved 2 snippets from Verilog to SystemVerilog, as they are not compatible with the Verilog-2005 standard, i.e. the typedef enum and typedef struct packed.
• I have added a verilog_systemverilog.snippets, which extends SystemVerilog, to be used with vhda/verilog_systemverilog.vim.
• I have added quite a few of my snippets to the verilog.snippets file.

TODO.
I still would want to rearrange the snippets into an alphabetical order in future commits, it really makes the navigation easier. Without any consistent ordering one needs to rely on search functionalities and jump around a file a lot chaotically. Maybe some thematic + alphabetic pattern could also be established.