wp_richedit_pre() may better have a filter hook for customization & i18n

[mani_monaj]

For some i18n purposes, We need to take control of the default text shows up in TinyMCE when a blank post/page starts, wp_richedit_pre() is called before TinyMCE wants to show content for some filterings, It also checks if it's a blank post or not, I added an filter hook to this function named "richedit_pre"

[skeltoac]

Good idea! I've modified it a bit to remove the empty paragraph as default. It was only added to improve RTL support but it hurts the experience for LTR users. Now anyone writing a plugin to turn on RTL is burdened with adding '<p> </p>' to any empty string.

[ryan]

(In [3284]) Add richedit_pre filter. Props mani_monaj. fixes #2047