return section_mru in theta theme and cut short_fn if it is too long

[yuukibarns]

Return section_mru in theta theme so that people can configure layout like this:

config = function()
	local theta = require("alpha.themes.theta")
	theta.header.val = require("alpha.fortune")
	theta.config.layout = {
		{ type = "padding", val = 2 },
		theta.header,
		{ type = "padding", val = 2 },
		theta.section_mru,
		{ type = "padding", val = 2 },
	}
	require("alpha").setup(theta.config)
end,

i.e., remove buttons. Note that we can't access theta.section_mru if not return it.