Better UX in container creation dialog

[tfuxu]

Guided section:

• Initial view should have the Create button grayed out
• If the container name is invalid, gray out the Create button and/or show a toast notification about the error

From File section:

• Only allow *.ini files

From URL section:

• Validate provided URL before allowing user to proceed. Show apply button in EntryRow and on apply signal check if there's a connection to a provided URL. If not, leave the Create button grayed out and show toast notification about the error